For a skill / spell that has a Bodybuilding defense, Bodybuilding affects the success chance of any harmful effect the skill applies. Other values, like damage, are not affected. More specifically, if BB is the Bodybuilding of the spell target, the success chance of the effect is multiplied by:

1 – 0.15 * BB (or 100% - 15% * BB). Here is an example:

Let's say a wizard with INT 12 is trying to freeze an enemy fighter using the Freezing Touch spell. This spell has a 100% base success chance and INT will raise it to 120% (see the skills page).

On the other side, let's say the fighter’s Bodybuilding is 2. This means that the success ratio of the effect is limited by a 1 – 0.15 * 2 = 0.7 factor.

The overall success chance of the Freezing Touch spell is the product of the two figures above, which is 120% x 0.7 = 84%. So the fighter finally has a 100 - 84 = 16% chance to avoid freezing. This last figure is reported in the statistics window at the bottom right corner of the screen (maybe +/- 1 because of rounding).